Add CompressOption.preset.

this can be used to "train" the model for shared contexts.
can be useful if the model is directly used for compression.

it is currently unclear that this can improve actual packing;
the observed improvement was "only" around 30B for samples
so the eventual decoder cost may outweigh that.

index.d.ts

@@ -94,6 +94,7 @@ export class DefaultModel implements LogisticMixModel {
 
 export interface CompressOptions extends AnsOptions {
     inBits: number;
+    preset?: number[];
     calculateByteEntropy?: boolean;
 }
 

index.mjs

@@ -394,9 +394,18 @@ export class DefaultModel extends LogisticMixModel {
 //------------------------------------------------------------------------------
 
 export const compressWithModel = (input, model, options) => {
-    const { inBits, outBits, precision, calculateByteEntropy } = options;
+    const { inBits, outBits, precision, preset = [], calculateByteEntropy } = options;
     const encoder = new AnsEncoder(options);
 
+    for (let offset = 0; offset < preset.length; ++offset) {
+        const code = preset[offset];
+        for (let i = inBits - 1; i >= 0; --i) {
+            model.predict();
+            model.update((code >> i) & 1);
+        }
+        model.flushByte(code, inBits);
+    }
+
     const byteProbs = [];
     for (let offset = 0; offset < input.length; ++offset) {
         const code = input[offset];
@@ -429,9 +438,18 @@ export const compressWithModel = (input, model, options) => {
 };
 
 export const decompressWithModel = ({ state, buf, inputLength }, model, options) => {
-    const { inBits } = options;
+    const { inBits, preset = [] } = options;
     const decoder = new AnsDecoder({ state, buf }, options);
 
+    for (let offset = 0; offset < preset.length; ++offset) {
+        const code = preset[offset];
+        for (let i = inBits - 1; i >= 0; --i) {
+            model.predict();
+            model.update((code >> i) & 1);
+        }
+        model.flushByte(code, inBits);
+    }
+
     const reconstructed = [];
     for (let offset = 0; offset < inputLength; ++offset) {
         let current = 0;